摘要:

Carbon solubility in a face centred cubic (fcc) Fe1-yNiyalloy lattice was analysed on the basis of statistical thermodynamics assuming negligible C—C interaction in the fcc Fe1-yNiylattice. The minimum carbon solubility was observed for values of y of ~ 0·6–0·7 and appeared to be ascribable to the minimum number of available interstitial sites for occupation by carbon atoms in the fcc Fe1-yNiylattice around this composition. Accordingly, the extent of stabilisation for carbon atoms in the fcc Fe1-yNiywas estimated to be a minimum around this composition. The observed variation for the carbon solubility and the extent of stabilisation of the carbon atoms in the fcc Fe1-yNiylattice with respect to the composition y appeared to be related to the stability of the ferromagnetic phase.MST/3251

ISSN:0267-0836    

DOI:10.1179/026708396790165867

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

The available data on secondary creep in creep tests and on maximum stress in hot deformation tests obtained forδandαferrite have been reviewed and expressed in the form of the equationϵ= K(σ/E)Pcsinh(Vσ/RT) exp(—Qc/RT) with pc= 2·5, V = 0·0013 m3mol−1, and K = 2·6×1017s−1, E and Qcbeing temperature dependent functions. The approximate range of validity is Z =ϵexp (Qc/RT)≈101—1011s−1. Together with the corresponding equation for austenite the data can be used to predict the viscoplastic behaviour of austenite—ferrite mixtures.MST/1538

ISSN:0267-0836    

DOI:10.1179/026708396790165902

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of theαandγphases. The structures were studied using a TEM. It was observed that the temperature TAand timeτAof the isothermal transformation significantly influence the nature of theαandγphases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of theαphase in the range of TAstudied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of theγphase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of TA, bainitic ferrite continually improves its crystal structure.MST/3104

ISSN:0267-0836    

DOI:10.1179/026708396790165821

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

An assessment of the experimental findings leads to the conclusion that optimal structural superplasticity results from grain/interphase boundary sliding—diffusion coupled flow. An analysis of the boundary sliding process is presented first. By suggesting that both regions I and IIa (lower stress range of region II) of superplastic flow result from sliding—diffusion coupled flow, and—treating mesoscopic (cooperative) boundary sliding as the rate controlling mechanism for optimal superplasticity, the stress, temperature, and grain size dependences of the strain rate of deformation are predicted. The above equation is then related to the stress exponent n (the inverse of the strain rate sensitivity index m). An analysis for determining the true activation energy for the rate controlling process is presented. Expressions for the distribution of internal stresses arising from sliding and the boundary viscosity are derived, and the presence of an initial unsteady region, predicted in an earlier analysis, is shown to be a natural consequence of this approach.MST/3074

ISSN:0267-0836    

DOI:10.1179/026708396790165920

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

Initiation of quasicleavage (QC) cracks in delayed fracture of hydrogen charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed fracture of an unnotched specimen is found not to be caused by intergranular cracking as is commonly found in notched specimens, but by QC cracking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specimen. Furthermore, the tensile component of stress causes a QC crack to grow in the plane of maximum tensile stress. The condition under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress.MST/3264

ISSN:0267-0836    

DOI:10.1179/026708396790165876

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

The nucleation of creep cavities was analysed using data from the literature for creep resistant low alloy and 12%Cr steels. The number of cavities per unit area was in most cases found to be a linear function of the creep strain, the cavity nucleation rate increasing linearly with increasing minimum creep rate. As a result, the cavity nucleation rate showed the same type of stress and temperature dependence as the strain rate, i.e. a power law function of the stress, where the stress is raised to a power close to the exponent in the creep rate equation (Norton's exponent). Simple models relating the number of cavities to strain and time as well as relating the cavity nucleation rate to time were derived. The theoretically estimated cavity numbers and cavity nucleation rate in relation to strain and time showed satisfactory agreement with the observations for both low alloy and 12%Cr steels.MST/3353

ISSN:0267-0836    

DOI:10.1179/026708396790165858

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

Biaxial in phase fatigue tests were carried out on thin walled tube specimens of alloy 800HT at ambient temperature. The loading modes included tension, torsion, and combined tension—torsion with a tensile/shear plastic strain range ratioΔϵp/Δγp = 31/2. The influence of effective strain amplitudes and biaxiality on the initial growth of fatigue cracks was investigated using the replica technique. The results indicated that the loading conditions strongly affected the growth rates of short cracks. In torsion the cracks grew significantly more slowly than under axial or biaxial loading. A mean tensile stress perpendicular to the shear crack promoted its growth and reduced the fatigue life. The growth of the cracks could be described by theΔJ integral for axial and biaxial loading; the integration predicted the fatigue life under axial and biaxial loading correctly. However, significantly conservative lifetime predictions were obtained for pure torsional loading sinceΔJ does not include crack closure and crack surface rubbing.MST/3234

ISSN:0267-0836    

DOI:10.1179/026708396790165812

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data on the growth of short fatigue cracks under various stress levels and stress ratios for the six principal specimen orientations. Short cracks usually initiated at the debonded interfaces between the matrix and the inclusions. If there was no debonding, cracking sometimes occurred in the inclusions. In the early stages, short cracks propagated by a mechanism of inclusion influenced growth. Under low stress levels, usually one short crack was initiated which dominated most of the fatigue life, while under high stress levels there was multicrack interaction.MST/3249A

ISSN:0267-0836    

DOI:10.1179/026708396790165830

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

The sliding wear behaviour of a leaded tin bearing bronze was investigated over a range of applied pressures and sliding speeds with respect to the influence of microconstituents such as lead on the wear response. Significantly high wear rates were found at the minimum sliding speed due to extensive microcracking. This was evinced by the formation of coarse debris and considerable subsurface/wear surface cracking. The (micro) cracking tendency of the alloy prohibited the occurrence of subsurface deformation. The absence of a lead film was primarily due to the lead particles being engulfed in the coarse debris. Higher sliding speeds led to increased frictional heating making the alloy matrix viscoplastic. This in turn greatly suppressed the tendency of the alloy to exhibit microcracking, thereby facilitating interaction between the materials of the mating surfaces through wear induced plastic deformation. As a result, a stable transfer layer formed on the specimen surface. Interestingly, the formation of a lead film on the wear surface was also observed under these conditions. The above factors were mainly responsible for the improved wear behaviour of the alloy at higher speeds. Finer debris formation, less surface and subsurface damage, and the presence of both a deformed and stable transfer layer and a lead film strongly supported these observations. Material removal mechanisms involved delamination of the undeformed subsurface region causing chipping off at the minimum sliding speed. Higher speeds, however, caused delamination of the transfer layer. In addition to adhesion, three body abrasion was found to contribute considerably towards material removal. The formation and stability of a transfer layer and the presence of a lead film are at least two major factors which control the wear behaviour of leaded tin bronzes. It is found that the above phenomena which provide improved wear characteristics occur only under specific sliding conditions.MST/3218

ISSN:0267-0836    

DOI:10.1179/026708396790165885

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

Compaction and permeability behaviour are important influences on the processing of composites for a range of manufacturing techniques including liquid composite moulding (LCM) and compression moulding. This paper describes an experimental study of the factors influencing these characteristics for two glass fibre reinforcement media used in LCM: continuous strand mat and an aligned fabric produced using warp knitting technology. The compaction relationships for the materials are presented as functions of process variables such as pressure, temperature, and forming rate. In plane permeability relationships measured using rectilinear and radial flow tests are presented for different fibre orientations and packing fractions. The results are discussed with reference to processing by resin transfer and structural reaction injection moulding.MST/3248

ISSN:0267-0836    

DOI:10.1179/026708396790165911

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor